fix: only delete valid buffers on alpha close

[nicbytes]

Hope you're all having a good day.

Encountered a problem when restoring sessions in AstroVim.

To see the problem:

1. Install AstroVim

2. Add the following to ~/.config/nvim/lua/user/plugins/user.lua:

   {
   "goolord/alpha-nvim",
   opts = function()
     local dashboard = require "alpha.themes.dashboard"
     require('alpha.term')
   
     dashboard.section.terminal.command = "cowsay I have herd you have a bug. Moo-tivating fix in progress."
     dashboard.section.terminal.width = 42
     dashboard.section.terminal.height = 16
     dashboard.section.terminal.opts.redraw = true
     dashboard.section.terminal.opts.window_config.zindex = 1
   
     dashboard.section.header.val = {
       "NeoVim",
     }
     dashboard.section.terminal.opts.hl = "DashboardHeader"
     dashboard.section.footer.opts.hl = "DashboardFooter"
   
     dashboard.config.layout = {
       { type = "padding", val = vim.fn.max { 2, vim.fn.floor(vim.fn.winheight(0) * 0.2) } },
       dashboard.section.terminal,
       { type = "padding", val = 2 },
       dashboard.section.header,
       { type = "padding", val = 3 },
       dashboard.section.buttons,
       { type = "padding", val = 3 },
       dashboard.section.footer,
     }
     dashboard.config.opts.noautocmd = true
     return dashboard
   end,
   },

3. Use the <Space>Sl remap to load a previous session and observe the error.